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[Constructor(unsigned long width, unsigned long height)]
interface WorkerCanvas {
  attribute unsigned long width;
  attribute unsigned long height;
  RenderingContext? getContext(DOMString contextId, any... arguments); 
  void toBlob(FileCallback? _callback, optional DOMString type, any... arguments);
  ImageBitmap transferToImageBitmap();

partial interface RenderingContext {
  void commit();

WorkerCanvas implements Transferable;
ImageBitmap implements Transferable;

partial interface HTMLCanvasElement {
  WorkerCanvas transferControlToWorker();

partial interface ImageBitmap {
  void transferToCanvas((HTMLCanvasElement or WorkerCanvas) canvas);
  void close();

Spec changes

Transferring of ImageBitmaps has to be defined. It should neuter the ImageBitmap in the sending thread. ImageBitmap.close also neuters the ImageBitmap. Neutering should set the ImageBitmap's width and height to 0.

HTMLCanvasElement.transferControlToWorker behaves like transferControlToProxy in the current WHATWG spec. WorkerCanvas is Transferable, but transfer fails if transferred other than from the main thread to a worker. All its methods throw if not called on a worker, or if it's neutered.

ImageBitmap.transferToCanvas replaces the canvas contents with the contents of the ImageBitmap, setting the canvas size to the ImageBitmap's size.